A turbomolecular pump, a vacuum pumping system and a method of evacuating a vacuum chamber

ABSTRACT

A turbomolecular pump, vacuum pumping system and method of evacuating a vacuum chamber is disclosed. The turbomolecular pump comprises: a rotor comprising a plurality of rotor blade rows, a stator comprising a plurality of stator blade rows and an outer casing, the rotor being rotatably mounted within the stator; wherein at least a portion of a surface of at least one of the rotor and the stator comprise a non-evaporable getter material.

CROSS-REFERENCE OF RELATED APPLICATION

This application is a Section 371 National Stage Application of International Application No. PCT/EP2020/064964, filed May 29, 2020, and published as WO 2020/239975 A1 on Dec. 3, 2020, the content of which is hereby incorporated by reference in its entirety and which claims priority of British Application No. 1907557.1, filed May 29, 2019 and British Application No. 2002394.1, filed Feb. 20, 2020.

FIELD

The field of the invention relates to a turbomolecular pump, a vacuum pumping system and a method of evacuating a vacuum chamber.

BACKGROUND

The ultimate pressure obtained by a turbomolecular pump is dependent on the partial pressure of the different gases and their compression ratio. The compression ratio for lighter molecule gases such as hydrogen is low and this impedes the ability of the pump to pump this gas. Where there is a mixture of gases, the lighter gases such as hydrogen are concentrated within the pump and pump exhaust conduit and this presence of lighter gases increases the ultimate pressure that the pump can achieve.

WO 2017/207706 discloses a vacuum device which may be a pump having at least one component having a portion in contact with the vacuum and which is coated at least in part by a non-evaporable getter (NEG) material. The material contributes to the maintenance of the vacuum and to reducing the pressure further. This document teaches its use in the inlet region of a turbomolecular pump. The document also taches that the NEG coating can be activated by heating the pump to 20° C. or partially activated by heating it to 80° C. prior to its use.

It would be desirable to be able to improve the ultimate pressure obtainable by a turbomolecular pump and in particular, one where lighter gases are being pumped.

The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.

SUMMARY

A first aspect provides a turbomolecular pump comprising: a rotor comprising a plurality of rotor blade rows, a stator comprising a plurality of stator blade rows and an outer casing, said rotor being rotatably mounted within said stator; wherein at least a portion of a surface of at least one of said rotor and said stator comprises a non-evaporable getter material; said turbomolecular pump further comprising: a pressure sensor for sensing pressure; a heater configured to heat said at least a portion of said turbomolecular pump such that said non-evaporable getter material is heated to above its activation temperature; and control circuitry for controlling operation of said turbomolecular pump said control circuitry being configured in response to a signal from said pressure sensor indicating a pressure to have fallen below a first predetermined value to activate said heater.

Different types of vacuum pumps have different properties and some pumps such as non-evaporable getter pumps have properties that are complementary to the properties of turbomolecular pumps, such that these two pumps are used together in some circumstances. However, the use of the two pumps together has the disadvantages of increased size and cost and also of additional obstructions in the fluid flow path leading to reduced conductance. The inventors of the present invention recognised that the disadvantages of combining the pumps could be overcome or at least alleviated were at least some of the surfaces of the turbomolecular pump to comprise a non-evaporable getter material, either where the surfaces are coated in such a material or where that portion of the pump is formed of that material. In this way the turbomolecular pump (TMP) would have the properties of a turbomolecular pump combined with at least some of the properties of a non-evaporable getter pump, thereby providing many of the advantages of both types of pumps.

In particular, non-evaporable getter material is very effective at capturing lighter gases such as Hydrogen and thus, can be used to improve the ultimate pressure obtainable by a conventional TMP. Furthermore, were the TMP to need to be stopped for a while due to a need for low vibrations or operation in a magnetic field, then the presence of the NEG material would help maintain the vacuum within the chamber and avoid the pressure rising unduly.

In addition the disadvantages of the NEG material being depleted due to the amount of gas being pumped which may rise due to outgassing and stimulated desorption effects, is mitigated by the TMP which does not have the same finite pumping capacity. This is particularly so, where the activating of the NEG material occurs only after a certain vacuum has been reached. The NEG material can initially be inactive and may only be activated by heating it above a certain temperature during operation. The ability to activate the material in this way inhibits it being exhausted prematurely by being active during initial stages of evacuation at higher pressures.

Furthermore, the NEG material may not be effective at capturing some inert molecules such as nitrogen, while the TMP may pump these very effectively. Thus, the combination of the properties provides a particularly effective pump and by at least some of the surfaces within the TMP pump comprising the NEG material a combination type pump is achieved with no or very minor additional obstruction to fluid flow and thus, conductance is not unduly reduced.

In some embodiments, said control circuitry is further configured to activate rotation of a rotor of said pump in response to a signal from said pressure sensor indicating a pressure to have fallen below an initial predetermined value, said first pressure being lower than said initial pressure.

The turbomolecular pump may not be activated until a certain low pressure is reached, and then once activated when a still lower pressure has been reached the turbopump may be heated to activate the NEG material.

In some embodiments, said control circuitry is further configured to deactivate said heater in response to one of: a predetermined time and a temperature sensor indicating a temperature in said turbomolecular pump being at or above a predetermined value.

In some embodiments, said control circuitry is configured in response to detecting said pressure reaching a second predetermined level, to generate a rotor deactivation signal for deactivating rotation of said rotor.

In some cases once a certain low pressure is reached, the rotor may be stopped and the vacuum maintained by the NEG material. This may be particularly advantageous where the pump is being used to evacuate a vacuum chamber used for a process that is vibration sensitive.

The at least a portion may vary but in some embodiments, said at least a portion comprises at least one of a surface of a subset of said rotor blade rows and an inner surface of said outer casing.

By providing existing surfaces within the pump with the NEG material the properties of a NEG pump are provided within the TMP pump without unduly obstructing the flow and lowering conductance.

In some embodiments, additional strips of material with NEG coatings on them may be introduced into the pumping system to further increase the NEG properties of the pump. This will affect conductance but will increase the surfaces available for the material and thus, depending on the implementation this may have advantages However, in other embodiments, it is preferable if the NEG material is provided only as a coating on existing surfaces of the TMP and on surfaces of conduits or pipes connecting the TMP with the vacuum chamber and/or downstream portions of the vacuum pumping system.

In some embodiments, the turbomolecular pump further comprises an exhaust conduit for exhausting gas output from said turbomolecular pump, at least a portion of an internal surface of said exhaust conduit being coated with a non-evaporable getter material.

As noted previously TMPs are not very effective at compressing lighter molecules and these molecules tend to become concentrated within the pump and pump exhaust. Thus, providing the NEG material as a coating within the exhaust will help the pumping capacity of the pump and improve the ultimate performance.

In some embodiments, said at least a portion of said exhaust conduit is detachably mounted to said turbomolecular pump.

One further advantage of coating the exhaust conduit is that such a conduit is more easily replaceable than portions of the TMP itself and thus, with a suitable detachable design, the exhaust conduit could be simply replaced when the NEG material is depleted or exhausted allowing it to be replenished and improving the performance of the pump over its lifetime.

In some embodiments, said turbomolecular pump comprises a heater configured to heat at least a portion of said turbomolecular pump such that said non-evaporable getter material is heated to above its activation temperature.

One advantage of NEG material is that it needs heat to activate it and thus, at the start of any pumping cycle where the pressure may be high the NEG material may not be active thereby avoiding it operating at these higher pressures and preserving its lifetime. Once the pressure has fallen to a suitably low value, the TMP may be heated and the NEG material activated. In this regard many conventional TMPs have a heating band such that during bakeout of the vacuum chamber the TMP is also heated to avoid or reduce temperature differentials. It may be that bakeout is a good opportunity to activate the material and the pressures at which bakeout is performed may be suitable for activation of the NEG material.

A second aspect provides a turbomolecular pump comprising: a rotor comprising a plurality of rotor blade rows, a stator comprising a plurality of stator blade rows and an outer casing, said rotor being rotatably mounted within said stator; wherein at least a portion of at least one of said stator and said outer casing comprises a gas capture structure for capturing gas, said gas capture structure comprising a skeletal framework comprising a non-evaporable getter material, said skeletal framework being formed from an aerogel.

Providing a turbomolecular pump with capture material in the form of a non-evaporable getter material in a structure that increases the surface area available for capturing gas molecules increases the performance and lifetime of the pump. As the pumping speed/capacity of the a pump depends at lower pressures on the surface area of the capture material, increasing the surface area increases the pumping speed/capacity.

An aerogel is a substance with a very high porosity and corresponding high surface area and as such forms a particularly effective structure for such a gas capture material allowing a very high surface area per unit volume and per unit mass of the capture material.

An aerogel is the dry, porous solid framework of a gel. It has an ultralow density, and is formed of the part of a gel that provides its solid-like cohesiveness. This solid portion is isolated from the gel's liquid component when forming the aerogel. As the liquid component takes up much of the volume of the gel the resulting structure is very porous and provides a large surface area available for the capture of gas molecules. In this way a significantly increased surface area can be provided within a set volume. It is the surface area that provides the capture sites and thus, the pumping speed and lifetime of a pump of a given size and with a given amount of capture material can be significantly increased when compared a conventional surface.

In some embodiments, said skeletal framework is coated with said gas capture material. While in other embodiments, said skeletal framework comprises said gas capture material, said aerogel being formed from said gas capture material.

As noted previously the aerogel forms a structure with a very high surface area owing to its porous nature. Where this surface area comprises the gas capture substance the amount of gas capture that can be provided by a certain volume is significantly increased. The gas capture material may be coated on an aerogel structure that is formed of a different material or in some embodiments the aerogel may be formed of the gas capture material itself. Forming the structure of the gas capture material obviates the need for a separate support structure and can be an efficient means of forming the gas capture structure. Furthermore, the porosity of the final product may be higher if there is no coating on the aerogel.

In some embodiments, said skeletal framework comprises an open cellular structure having a porosity of more than 60%, in some cases more than 70%.

In some embodiments said at least a portion comprises a static portion of said turbomolecular pump close to an inlet of said pump.

In some embodiments, wherein said at least a portion comprises a spider for mounting said rotor at an inlet of said turbomolecular pump.

It may be preferable for the portion comprising the non-evaporable getter material to be close to the inlet which experiences the lower pressures. It may be also advantageous if it is the static surfaces such as the stator, spacers or spider at the inlet that are formed of the aerogel material.

A third aspect provides a vacuum pumping system comprising a turbomolecular pump according to a first or second aspect, and comprising at least one further pump downstream and in series with said turbomolecular pump.

It may be advantageous to have one or more further pumps downstream and in series with the turbomolecular pump. These further pumps may comprise a primary pump for initial pump down of the chamber and to act as a backing pump for the turbomolecular pump. In this regard, the primary pump may reduce the pressure within the chamber and within the turbomolecular pump prior to this pump being operational. The turbomolecular pump may then be started and after a period, the turbomolecular pump may be heated to activate the non evaporable getter material.

In some embodiments, said at least one further pump comprises a further turbomolecular pump.

It may be particularly effective for the at least one further pump to be a turbomolecular pump. The turbomolecular pump of an embodiment provides effective pumping and reduces the lighter gases in the exhaust, a further TMP can then provide additional compression of the gas stream. It may also be used to help provide a high vacuum prior to activating the getter material in the first TMP thereby increasing the lifetime of this material.

In some embodiments the at least one further pump is connected to the turbomolecular pump via the exhaust conduit.

A fourth aspect provides a method of evacuating a chamber comprising: attaching a vacuum pumping system according to a first aspect to said chamber;

evacuating said chamber to a first pressure using said primary pump; starting the turbomolecular pump; activating a heater to heat said turbomolecular pump to activate said non-evaporable getter material; de-activating said heater.

In order to evacuate a chamber using a vacuum pumping system according to an embodiment the system is attached to a vacuum chamber and the chamber is evacuated to a first pressure using the primary pump. Once the first pressure has been attained the turbomolecular pump is started and the pressure is reduced still further. At a certain point a heater is activated to heat the turbomolecular pump and this activates the non-evaporable getter material. At a certain point once the getter material has reached a temperature sufficient to activate it, the heater will be switched off and the pump will continue to pump. The system continues pumping with the pumping operation of the turbomolecular pump being combined with that of the getter material and thereby providing effective pumping to the gases including any lighter gasses such as hydrogen.

In some embodiments, the method comprises a further step of stopping rotation of said turbomolecular pump and continuing to pump said vacuum chamber using capture by said non-evaporable getter material.

In some cases it may be advantageous to stop rotating the turbomolecular pump, for example where the turbomolecular pump is used in an analyser and vibrations are detrimental to an analysing step or where the magnetic field of the magnetic bearings may affect or be affected by such an analyser. Stopping the turbomolecular pump according to an embodiment will allow the vacuum to be maintained to an acceptable degree as the capture materials will still be active and will continue to provide their own pumping.

In some embodiments, said heating step is part of a bakeout process during which the vacuum chamber and turbomolecular pump are heated to encourage outgassing.

Although the heating step may be a separate step simply to activate the getter material when the pressure has dropped below a certain value such that the getter material is not exhausted by capturing molecules when the pressure is higher, in some cases it may be part of a bakeout process during which the vacuum chamber and turbomolecular pump are heated to encourage outgassing. In this regard, vacuum chambers are often heated to encourage outgassing and reduce contamination and in many cases the turbomolecular pump is heated at the same time such that there is not an unacceptable level of condensation on the blades of the turbomolecular pump when the hot gas from the chamber is evacuated through the turbomolecular pump. Such a bakeout step is a convenient way of activating the getter material and provides a further advantage of this system.

It should be noted that when the getter material has become exhausted or is close to exhaustion then the pumping system will still operate as a conventional turbomolecular pump and may still provide an acceptable vacuum.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

The Summary is provided to introduce a selection of concepts in a simplified form that are further described in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

FIG. 1 shows a vacuum pumping system according to an embodiment;

FIG. 2 shows a vacuum pumping system according to a further embodiment; and

FIG. 3 shows a flow diagram illustrating steps in a method according to an embodiment.

DETAILED DESCRIPTION

Before discussing the embodiments in any more detail, first an overview will be provided.

The ultimate pressure achieved by a turbomolecular pump (TMP) even under zero flow conditions, is limited by the partial pressure in the exhaust line and the compression ratio of the TMP for Hydrogen. Furthermore, the vacuum level can be quickly degraded in situations requiring the TMP to be stopped (for example during a vibration sensitive procedure or when a high magnetic field is applied).

Embodiments of the present invention address these problems with the use of a NEG coating activated at HV/UHV (high vacuum and ultra-high vacuum) pressures, this allows the ultimate and H₂ pumping performance of the TMPs to be significantly enhanced.

NEGs are known for their high pumping efficiency for H₂. TMPs are however, poor at pumping H₂. Coating at least some of the internal surfaces of the TMP such as the blade rows and the upper envelope with NEG material provides a pump that combines the pumping efficiency of a NEG with that of a TMP. The NEG may be activated at the same time as the bakeout of the vacuum system and TMP.

With TMPs, at pressures below 1×10⁻⁷ mbar the dominant gas load is increasingly H₂ and this is the limitation for ultimate performance of TMPs in UHV. The pumping speed for H₂ of a NEG coating is @ 0.35 l/s/cm² (c.f. 3 l/s cm² for a metal strip). For an ISO160 flange TMP with intrinsic H₂ pumping speed of ˜300 l/s each cross-sectional area NEG coating of ˜200 cm² would provide ˜70 l/s.

A coating of ˜3 blade rows+1×the inner envelope would give a NEG H₂ speed of ˜280 l/s.

The following advantages may be provided by embodiments:

1. Effective doubling of a similar pure TMP's H₂ pumping speed 2. Allows H₂ pumping to continue if the TMP is stopped 3. Effectively reduces the partial pressure of H₂ in the vacuum chamber, TMP and backing line. This will effectively ‘increase’ the H₂ compression ratio of the TMP or in time improve the ultimate pressure rather than degrade the ultimate pressure as currently happens in TMP systems where the H₂ partial pressure in its backing line rises with time

The NEG/TMP combination pump would be most suited for UHV and XHV (extremely high vacuum) situations where the activation of the NEG is performed during the heating of the chamber and TMP during bakeout.

The unlimited capacity of the TMP for outgassing and stimulated desorption loads overcomes a limitation of the finite pumping capacity of NEGs

FIG. 1 show as turbomolecular pump 10 according to an embodiment. This turbomolecular pump comprises a rotor 12 and a stator 14. In this embodiment the upper rotor blade surfaces and the inner surface of the stator 14 are coated with a non-evaporable getter material. In other embodiments, the lower rotor blades and the stator blades may also be coated with the NEG material.

The turbomolecular pump 10 has flange 15 for attaching to a vacuum chamber. TMP 10 also has a heating band 20 around its upper surface, which is used to activate the NEG material and this is switched on during a bakeout process of the vacuum chamber.

TMP 10 is attached via a conduit 35 to a valve 38 and then via a further conduit 36 to a primary pump 30. In some embodiments one or more of the conduits 35, 36 are also coated with the NEG material.

Primary pump 30 acts to evacuate the vacuum chamber to a first pressure, whereupon the TMP 10 is activated and evacuates the vacuum chamber to a second pressure. The primary pump 30 continues to run and operates as a backing pump to the TMP. When the second pressure has been reached, the heater 20 is switched on and the non-evaporable material is activated, whereupon the turbomolecular pump operates both as a pump both as a molecular pump and as a capture pump.

In this embodiment control circuitry 45 associated with turbomolecular pump 10 receives signals from a pressure sensor 42 and controls the TMP 10 to perform the functions described above. In particular, when pressure sensor 42 indicates the pressure in the TMP 10 has fallen to the first pressure the control circuitry 45 activates the rotor 12 and when the pressure has fallen to the second pressure the control circuitry activates heater band 20. When a temperature sensor (not shown) indicates that the temperature within the pump has risen to the activation temperature of the non-evaporable getter material the control circuitry 20 sends a signal to turn the heater band off. When the pressure sensor 42 indicates that a further low pressure has been reached in some embodiments the control circuitry may control the rotor to stop rotating, at which point the vacuum within the vacuum chamber may be maintained by the activated NEG material.

In this embodiment conduit 36 connecting the two pumps is itself coated with a getter material which can also be activated by heating. The conduit 36 is removably connected to the turbomolecular pump 10 such that when the non-evaporable getter material of the exhaust is depleted the exhaust conduit 36 can be exchanged for one with fresh non-evaporable getter material on it allowing this material to be activated on heating and provide additional capture. In this regard, conduit 36 is attached via a valve mechanism 38 such that the vacuum within the chamber and turbomolecular pump can be maintained during the exchange of the exhaust conduit 36.

In other embodiments the NEG material may be coating or formed of an aerogel, which aerogel structure may form portions of the static part of the TMP 10, particularly those adjacent to the inlet of the pump. In some embodiments, portions of the stator, the blade spacers and/or the spider may be formed of an aerogel.

FIG. 2 shows a further embodiment which is similar to that of FIG. 1 but this has an additional turbomolecular pump 40 arranged downstream of turbomolecular pump 10. Turbomolecular pump 40 is also backed by primary pump 30.

There are a plurality of valves 38 a, 38 b, 38 c allowing the different pumps to be connected together in different arrangements. In this embodiment, conduit 37 between the two turbomolecular pumps 10, 40 is coated with NEG material and is detachably connected to the vacuum system allowing it to be replaced. Valve 38 a can be closed during replacement of the conduit 37 and in some embodiments TMP 10 can continue to operate backed by primary pump 30. Following replacement of conduit 37, valve 38 c may be opened and 38 b closed allowing pressure in TMP 40 and conduit 37 to be reduced, whereupon TMP 40 can be switched on, and at a certain point conduit 37 heated and valve 38 a opened. The additional TMP 40 helps reduce the pressure further and owing to the NEG material in TMP 10 and 37 predominantly pumps gas from which the lighter gases have been substantially removed making for better compression.

FIG. 3 shows a diagram illustrating steps in a method for evacuating a chamber according to an embodiment. The evacuation of the chamber starts with the primary pump pumping down at step S10. When a pressure sensor senses the pressure to have reached a first pressure at step D05, the first pressure may be between 0.1 and 10⁻³ mbar, the TMP is activated at step S20 and pumps the chamber down to a second pressure. When the pressure sensors sense the pressure has fallen to the second pressure at step D15, which second pressure may be in the region of 10⁻⁷ to 10⁻⁹ mbar, the heater is activated at step S30. The getter material is activated by heating the TMP to the activation temperature of the material, this may occur at the same time as bakeout of the vacuum chamber.

Once the NEG material has been activated and a temperature sensor indicates that a temperature above the activation temperature has been reached at D25, then the heater is switched off at step S40 and the pump continues to pump until a lower pressure is sensed to have been reached at D35. In some embodiments where the vacuum pump is used to evacuate a chamber of an analyser, this third pressure will be the pressure required for the analysis. Such an analyser may be an electron microscope or a mass spectrometer. Once the pressure required for analysis has been reached, in some embodiments the TMP and vacuum chamber may be isolated using a valve 38 of FIG. 1 and the TMP and primary pump may be switched off at step S50. This may occur where the analyser is vibration sensitive. The analyser may then be switched on at step S60, analysis performed and when complete the pumps may be switched on again and valve 38 opened. While the TMP is not rotating it will still be providing some pumping by the capture of gas molecules on the NEG material within the pump.

It should be noted that the vacuum system may be controlled to perform the steps of this method by the control circuitry 45 of FIG. 1.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims. 

1. A turbomolecular pump comprising: a rotor comprising a plurality of rotor blade rows, a stator comprising a plurality of stator blade rows and an outer casing, said rotor being rotatably mounted within said stator; wherein at least a portion of a surface of at least one of said rotor and said stator comprises a non-evaporable getter material; said turbomolecular pump further comprising: a pressure sensor for sensing pressure; a heater configured to heat said at least a portion of said turbomolecular pump such that said non-evaporable getter material is heated to above its activation temperature; and control circuitry for controlling operation of said turbomolecular pump said control circuitry being configured in response to a signal from said pressure sensor indicating a pressure to have fallen below a first predetermined value to activate said heater.
 2. The turbomolecular pump according to claim 1, wherein said control circuitry is further configured to activate rotation of a rotor of said pump in response to a signal from said pressure sensor indicating a pressure to have fallen below an initial predetermined value, said first pressure being lower than said initial pressure.
 3. The turbomolecular pump according to claim 1, wherein said control circuitry is further configured to deactivate said heater in response to one of: a predetermined time and a temperature sensor indicating a temperature in said turbomolecular pump being at or above a predetermined value.
 4. The turbomolecular pump according to claim 1, wherein said control circuitry is configured in response to detecting said pressure reaching a second predetermined level, to generate a rotor deactivation signal for deactivating rotation of said rotor.
 5. The turbomolecular pump according to claim 1, wherein said at least a portion comprises at least one of a subset of said rotor blade rows and an inner surface of said outer casing.
 6. The turbomolecular pump according to claim 1, further comprising an exhaust conduit for exhausting gas output from said turbomolecular pump, at least a portion of an internal surface of said exhaust conduit being coated with said non-evaporable getter material.
 7. The turbomolecular pump according to claim 6, wherein said at least a portion of said exhaust conduit is detachably mounted to said turbomolecular pump.
 8. A turbomolecular pump comprising: a rotor comprising a plurality of rotor blade rows, a stator comprising a plurality of stator blade rows and an outer casing, said rotor being rotatably mounted within said stator; wherein at least a portion of at least one of said stator and said outer casing comprises a gas capture structure for capturing gas, said gas capture structure comprising a skeletal framework comprising a non-evaporable getter material, said skeletal framework being formed from an aerogel.
 9. The turbomolecular pump according to claim 8, wherein said at least a portion comprises a static portion of said turbomolecular pump close to an inlet of said pump.
 10. A vacuum pumping system comprising a turbomolecular pump according to claim 1, and comprising at least one further pump downstream and in series with said turbomolecular pump.
 11. The vacuum pumping system according to claim 10, wherein said at least one further pump comprises a further turbomolecular pump.
 12. The vacuum pumping system according to claim 10, wherein said at least one further pump comprises a primary pump.
 13. A method of evacuating a chamber comprising: attaching a vacuum pumping system according to claim 12 to said chamber; evacuating said chamber to a first pressure using said primary pump; rotating the rotor of the turbmolecular pump; activating a heater to heat said turbomolecular pump to activate said non-evaporable getter material; deactivating said heater.
 14. The method according to claim 13, comprising a further step of stopping rotation of said turbomolecular pump and continuing to provide a pumping process using capture by said non-evaporable getter material.
 15. The method according to claim 13, wherein said heating step is part of a bakeout process during which the vacuum chamber and turbomolecular pump are heated to encourage outgassing. 